There are presently programs directed toward developing high data rate, nominally 300 MBps to 1 GBps, optical communication systems for space flight use. A typical system might employ a Nd:YAG mode locked laser transmitter using on-off key (OOK), binary polarization (BPM) or pulse quaternary (PQM) modulation formats and a photoemissive direct detection receiver incorporating either high speed, gated photomultiplier tubes or avalanche photodiode-low noise, wideband amplifier combinations. Such systems are governed by Poisson statistics. The performances thereof for OOK or BPM modulation formats and deterministic average receiver signal intensity have previously been analyzed and reported in the literature.
Optical communication systems appear at present most attractive in the space-to-space link. In the space-to-space configuration, it is easiest to take full advantage of the relatively small size, power and weight of optical communication systems as well as the rugged and long lifetime components used in such systems. However, in the space-to-space configuration, as exemplified by a low earth orbiter to synchronous satellite link, the operational system may require sub-arcsecond pointing of the laser transmitter beam. As a result, a dominant noise source may be the interaction of the transmitter far field irradiance profile and the instantaneous transmitter pointing direction. Thus, the average receiver signal level may be random and obey beta statistics, as reported in the literature. For the space-to-ground link one can take advantage of the transmitter low power, light weight, etc. system parameters, but must contend with the perturbing effects of the atmosphere. Performance analysis reported in the literature for the space-to-ground link incorporate the effects of atmospheric scintillation assuming a log normal channel. Theory exists and has been reported asserting that certain space-to-ground optical communication links will also be subject to atmospheric scintillation resulting in time varying average signal amplitudes described by Ricean or Rayleigh probability distributions.
Past attempts to simulate the random variations of space-to-earth and space-to-space optical communication links have involved physically perturbating air between a source of optical radiation and a detector for the radiation. The air has generally been perturbated by heating or introducing air current turbulence. It has been found that such techniques do not provide sufficiently consistent and accurate simulation of the random variations to enable laboratory testing of the link in a desired manner.